Portable gas sensor and method for calibrating the same

ABSTRACT

The present invention provides a portable gas sensor having a sensing material reacted with a target gas. The portable gas sensor includes a mode input unit for selecting an operation mode; a gas inhaling unit for accepting an external gas; a sensing unit for outputting a voltage value corresponding a varied resistance value of the sensing material reacted with the external gas; a first memory for storing a reference voltage value corresponding a resistance value of the sensing material reacted with a reference gas; a calibration control unit for storing a first voltage value corresponding to a resistance value of the sensing material reacted with a substitute reference gas in the first memory by substituting the reference voltage value; a target gas sensing control unit for storing a second voltage correspond to a resistance value of the sensing material reacted with the target gas into the second memory; a comparison and calculation unit for comparing and calculating the first voltage value and the second voltage value; and a display unit for displaying a concentration of the target gas calculated in the comparison and calculation unit.

TECHNICAL FIELD

The present invention relates to a gas sensor; and, more particularly,to a portable gas sensor and a method for calibrating the same.

BACKGROUND ART

As industrial developments have been accelerated ever since theIndustrial Revolution, problems related to atmospheric pollution causedby using such hazardous gases as carbon monoxide (CO), hydrogen sulfide(H₂S), sulfur dioxide (SO₂) and nitrogen oxide (NO_(x)) have beenincreasingly issued. Also, danger of gas explosion and gas poisoning hasbeen alerted.

After the first report on a catalytic combustion-type sensor by Johnson,various types of the gas sensor have been developed. Particularly, asemiconductor-type gas sensor, first discovered by Seiyama and Taguch,detects the presence or absence of a specific gas and its concentrationthrough estimation of changes in resistance by using sintered materials,e.g., tin dioxide (SnO₂), zinc oxide (ZnO), and indium trioxide (In₂O₃),which are metal-oxide semiconductor materials. The semiconductor-typegas sensor was first commercialized in 1968 by the company, Figaro, inJapan and has been used mainly in a gas leakage alarming system and agas concentration measurement system.

In the course of developing various types of the gas sensor, they havebeen modified to meet the purpose of use and detection of a certaindesired gas: through improvements on employed materials and applicablesensing apparatuses. As a result of these efforts, various types of thegas sensor have been commercialized to be used in industries, medicalfields and daily lives. Particularly, wide distribution of liquefiednatural gas (LNG) and liquefied petroleum gas (LPG) and public attentionto socially issued problems of drunk driving has led the gas sensor tobe further commercialized in different types such as a portable leakinggas sensor or a portable alcohol analyzer.

Semiconductor is classified into an n-type and a p-type depending onconductivity mechanisms. Tin dioxide (SnO₂), which is the most typicalsensitizer, is one of the n-type semiconductors. The number of positiveions of Sn is quantitatively less than that of negative ions of O andthus, producing unpaired electrons which contribute to a degree ofconductivity. This SnO₂ then adsorbs oxygen in atmosphere to balance thenumber of positive and negative ions. Because of anionic characteristicsof the adsorbed oxygen, electrons contributing to the conductivity ofthe semiconductor become locally captured in a surface of the adsorbedoxygen. As a result of this capturing state, the electrons lose theirconductivity.

If the SnO₂ with the adsorbed oxygen is exposed to a reduction gas,e.g., CO and ammonia (NH₃), the adsorbed oxygen react with suchreduction gas and become desorbed from the SnO₂ as shown in thefollowing chemical equation.2CO+O₂→2CO₂  Eq. 1As shown in the Eq. 1, the captured electrons become free and contributeto the conductivity. Therefore, a specific gas type to be detecteddetermines a degree of the conductivity of a semiconductor sensor. Thedetection of changes in the conductivity degree provides information onthe presence or absence of a specific desired gas and its concentrationif presents. Such material as platinum (Pt), gold (Au) and silver (Ag)are also added to the sensitizer SnO₂ as a catalyst in order to increasea sensing ability.

For a conventional portable gas sensor, there exists a problem offrequent errors caused by a different operating mechanism from a gasalarm which is continuously operated for 24 hours a day and 365 days ayear. That is, the portable alcohol analyzer or portable leaking gassensor is operated only in need. However, this type of the gas sensoralso reacts with undesired gases depending on external environmentalfactors, e.g., temperature, humidity, atmospheric pressure andsubsequently changes a reference value of the gas sensor. As a result,measurements by the gas sensor become deviated in a greater extent.

FIG. 1 is a graph showing changes in resistance depending on aconcentration of alcohol in a portable alcohol analyzer. Typically, therelationship between the resistance change and the alcohol concentrationin the conventional semiconductor gas sensor is expressed algebraically.However, this algebraic relationship is expressed linearly in FIG. 1.

Assuming that the conventional semiconductor-type gas sensor followscharacteristics of the curve I shown in FIG. 1, the resistance iscalibrated to about 20 KΩ when about 80 PPM of alcohol gas is injectedinto the gas sensor. Based on this calibration, a correspondent gasconcentration can be easily determined through interpolation with theknown measured resistance value.

FIG. 2 is a circuit diagram showing equivalent circuits of a bulk-typesemiconductor gas sensor. A resistance value of a sensing material isdifferent from each sensor. As shown in the equivalent circuits in FIG.2, although properties of a sensing material vary corresponding toexternal environmental factors such as temperature and humidity, outputvalues of the gas sensor are set to be identical with use of a variableresistance VR during an initial fabrication of the above gas sensor.Once the resistance values are calibrated to be identical under apredetermined consistent gas concentration by using the variableresistance VR, a relative gas concentration can be easily determined.The reference notations ‘Vh’, ‘Rh’, ‘Rs’, ‘Vcc’ and ‘Vout’ express aheater voltage, a heater resistance, a signal resistance, a powervoltage and an output voltage, respectively.

However, accuracy of the gas sensor decreases if characteristics of thegas sensor change from the curve I to the curve II or III as aresistance value of the sensing material changes by the externalenvironmental factors. For example, if the characteristic of the gassensor is changed from the curve I to the curve II, the resistance valueof the gas sensor is about 10 KΩ with respect to about 80 PPM of thealcohol gas and a calibrated concentration of the alcohol gas is about320 PPM. Meanwhile, if the characteristic of the gas sensor followingthe curve I changes to the curve III, the resistance value of the gassensor is about 30 KΩ with respect to about 80 PPM of the alcohol gasand the alcohol gas concentration is expressed in about 20 PPM based onthe reference calibration. That is, there exists a high risk ofdeviations in the detection result according to changes incharacteristics of the gas sensor.

In that case, another calibration is required. However, it is difficultto obtain a conveniently applicable reference gas, and the calibrationproceeds manually by a manufacturer since the calibration skill has notbeen developed yet.

DISCLOSURE OF THE INVENTION

It is, therefore, an object of the present invention to provide aportable gas sensor allowing a convenient calibration by a user and amethod for calibrating the same.

In accordance with an aspect of the present invention, there is provideda portable gas sensor having a sensing material reacting to a targetgas, including: a mode input unit for selecting an operation mode; a gasinjecting unit bringing in an external gas; a sensing unit outputting avoltage value corresponding to a changed resistance value of the sensingmaterial due to a reaction between the sensing material and the injectedexternal gas; a first memory storing a reference voltage valuecorresponding to a resistance value of the sensing material with respectto a reference gas; a calibration control unit for storing a firstvoltage value in the first memory by replacing the reference voltagevalue stored in the first memory through an operation of a calibrationmode initiated in response to an input from the mode input unit, thefirst voltage value corresponding to the changed resistance value of thesensing material reacting with a substitutionary reference gas; a targetgas sensing control unit for storing a second voltage value in a secondmemory by operating a target gas measurement operation mode initiated inresponse to an input of the mode input unit, the second voltage valuecorresponding to a changed resistance value of the sensing materialreacting with the target gas transferred through the gas injecting unit;a comparison and calculation unit for comparing the first voltage valuewith the second voltage value and estimating the comparison value; and adisplay unit displaying a concentration of the target gas estimated fromthe comparison and calculation unit.

In accordance with another aspect of the present invention, there isalso provided a method for calibrating a portable gas sensor having asensing material reacting to a target gas, including the steps of:storing a reference voltage value corresponding to a resistance value ofthe sensing material with respect to a reference gas in a memory;inputting a calibration operation mode by maneuvering a key operation;measuring a fist voltage corresponding to a resistance value of thesensing material with respect to a substitutionary reference gas; andstoring the first voltage value in the memory by replacing the storedreference voltage value.

In accordance with still another aspect of the present invention, thereis also provided a computer readable recording medium storinginstructions for implementing the method for calibrating the portablegas sensor having the sensing material reacting to the target gas, thecomputer readable recording medium including the instructions of:storing a reference voltage value corresponding to a resistance value ofthe sensing material with respect to a reference gas; inputting acalibration operation mode by maneuvering a key operation; measuring afist voltage corresponding to a resistance value of the sensing materialwith respect to a substitutionary reference gas; and storing the firstvoltage value in the memory by replacing the stored reference voltagevalue.

BRIEF DESCRIPTION OF THE DRAWINGS

Other objects and aspects of the invention will become apparent from thefollowing description of the embodiments with reference to theaccompanying drawings, in which:

FIG. 1 is a graph showing changes in resistance depending on aconcentration of alcohol gas in a conventional portable alcoholanalyzer;

FIG. 2 is a circuit diagram showing equivalent circuits of aconventional bulk-type semiconductor gas sensor;

FIG. 3 is a block diagram of a portable gas sensor in accordance with apreferred embodiment of the present invention;

FIG. 4 is a graph illustrating an example of a lookup table for a bloodalcohol-concentration (BAC) in accordance with the preferred embodimentof the present invention; and

FIG. 5 is a flowchart showing a calibration operation procedure and ameasurement operation procedure of the portable gas sensor in accordancewith the preferred embodiment of the present invention.

BEST MODE FOR CARRYING OUT THE INVENTION

Hereinafter, a preferred embodiment of a portable gas sensor allowing aconvenient calibration by a user and a method for calibrating the samewill be described in detail referring to the accompanying drawings.

FIG. 3 is a block diagram showing the portable gas sensor in accordancewith the preferred embodiment of the present invention.

Referring to FIG. 3, the portable gas sensor includes a mode input unit30, a calibration control unit 32, a target gas sensing control unit 34,a gas injecting unit 36, a sensing unit 38, a first memory 40, a secondmemory 42, a comparison and calculation unit 44, a third memory 46 and adisplay unit 48. A user selects a calibration operation mode or a targetgas measuring operation mode of the portable gas sensor through the modeinput unit 30. A gas from an external source is injected into the gasinjecting unit 36, and the injected gas reacts with a sensing materialof the sensing unit 38. Because of the chemical reaction between theinjected gas and the sensing material, a resistance value of the sensingmaterial changes and a voltage value correspondent to the changedresistance value is stored into the first memory 40 or the second memory42 depending on the input of the mode input unit 30.

In accordance with the preferred embodiment of the present invention,during an initial manufacturing procedure, the first memory 40 is storedwith a reference voltage value corresponding to a resistance value ofthe sensing material with respect to a reference gas. Afterwards, oncethe calibration operation mode is operated, a voltage value outputtedfrom the sensing unit 38 is stored as a substitutionary referencevoltage value under a control of the calibration control unit 32. Also,the second memory 42 stores an outputted voltage value from the sensingunit 38 under a control of the target gas sensing control unit 34. Thatis, the voltage value stored into the first memory 40 changes its valueonly when the calibration operation is performed and is then stored in areflected value of this resistance change. The voltage value stored intothe second memory is stored in a different value as the target gas ismeasured.

The comparison and calculation unit 44 compares the voltage valuesstored in the first memory 40 and the second memory 42 with each other,and the comparison value is then calculated into a concentration of thetarget gas by using a lookup table stored in the third memory 46. Theestimated concentration is seen in a liquid crystal display (LCD)through the display unit 48.

Hereinafter, the calibration operation and the target gas measuringoperation according to the present invention will be explained in moredetail.

First, a user selects the calibration operation mode through the modeinput unit 30. Then, the sensing material of the sensing unit 38 isheated up to a predetermined temperature for a specific time forinitialization. After the initialization state is reached through theheat treatment to the sensing material, the sensing unit 38 senses achange in the resistance of the sensing material due to a chemicalreaction between the sensing material and a substitutionary referencegas transferred from the gas injecting unit 36. Then, under a control ofthe calibration control unit 32, a fist voltage value corresponding tothe changed resistance value is stored into the first memory 40 byreplacing the previously stored voltage value.

In addition to the calibration mode operation, a user can also selectthe target gas measuring operation mode through the mode input unit 30.Then, the sensing material of the sensing unit 38 is heated up to apredetermined temperature for a specific time for initialization. Afterthe initialization state is reached, the sensing unit 38 senses a changein resistance of the sensing material due to a chemical reaction betweenthe sensing material and a target gas transferred from the gas injectingunit 36. Then, under a control of the target gas sensing control unit34, a second voltage value corresponding to the changed resistance valueis stored into the second memory 42 by replacing the previously storedvoltage value. The comparison and calculation unit 44 compares the firstvoltage value stored into the first memory 40 with the second voltagevalue stored into the second memory 42, and the comparison value is thencalculated into a concentration of the target gas by using the lookuptable stored into the third memory 46. This estimated concentration ofthe target gas is seen in the LCD through the display unit 48.

FIG. 4 is a graph showing an exemplary lookup table with respect to ablood alcohol concentration (BAC) in accordance with the preferredembodiment of the present invention. Values in a bottom portion of thelookup table express a voltage difference between a first voltage valueand a second voltage value, while values in an upper portion of thelookup table express a BAC corresponding to the voltage difference.

Referring to FIG. 4, in case that the voltage difference between thefirst voltage value and the second voltage value is less than about 0.22V, the BAC of about 0.00% is shown in the LCD through the display unit48. This result is because even a breath without containing alcohol canhave a voltage difference in about 0.22 V. For instance, if the voltagedifference between the first voltage value and the second voltage valueis about 0.42 V, the BAC of about 0.002% is shown in the LCD of thedisplay unit 48. The values of the lookup table shown in FIG. 4 areexperimental data and can be applied differently depending on eachtarget gas type.

FIG. 5 is a flowchart showing a calibration operation procedure and ameasurement operation procedure of the portable gas sensor in accordancewith the preferred embodiment of the present invention.

More specifically, the calibration operation mode is initiated once auser who wants to use the calibration operation mode presses anexternally disposed calibration operation mode button of the mode inputunit 30 in the portable gas sensor. If the user wants to use a targetgas concentration measuring operation mode, he/she presses themeasurement operation mode button of the mode input unit 30 to initiatethe measuring procedure. In the preferred embodiment of the presentinvention, the portable gas sensor does not have separate buttons foreach operation mode but have one power button that differentiates thecalibration operation mode and the measurement operation mode by howlong the power button is pressed. That is, the calibration operationmode is selected by pressing the power button with above a predeterminedtime, and if otherwise, the target gas measurement operation mode isselected.

Referring to FIG. 5, the portable gas sensor is turned on by pressingthe power button at Step 10. Then, at Step 11, it is determined whetherthe power button is pressed with above a predetermined time. If thepower button is pressed for longer than the predetermined pressing time,the calibration operation mode is initialized at Step 12. Then, thesensing material is exposed to a substitutionary reference gas providedthrough the gas injecting unit 36 at Step 13. At this time, thesubstitutionary reference gas can be a human breath or air/atmosphere.The calibration control unit 32 recognizes the injected gas during thecalibration operation mode as a reference gas, e.g. a human breath.

Next, at Step 14, the calibration control unit 32 reads a first voltagevalue corresponding to a changed resistance value of the sensingmaterial from the sensing unit 38 by the injection of thesubstitutionary reference gas.

Thereafter, at Step 15, the calibration control unit 32 replaces thepreviously stored voltage value with the first voltage valuecorresponding to the changed resistance value of the sensing materialand stores the first voltage in the first memory 40. Then, the processis terminated.

If the power button is pressed for less than the predetermined pressingtime, the target gas measurement operation mode is initialized at Step21. Then, the sensing material of the sensing unit 38 is exposed to thetarget gas provided by the gas injecting unit 36 for a predeterminedtime at Step 22.

Next, at Step 23, the target gas measurement control unit 34 reads asecond voltage value corresponding to a resistance value of the sensingmaterial after the exposure of the sensing material to the target gas.

Subsequent to Step 23, the second voltage value is stored into thesecond memory 42 under a control of the target gas sensing control unit34 at Step 24. Then, the first voltage value and the second voltagevalue are compared with each other by the comparison and calculationunit 44 at Step 25. The resultant comparison value is substituted intothe lookup table stored in the third memory 46 to estimate aconcentration of the target gas corresponding to the comparison value.This estimated concentration is shown in the LCD of the display unit 4S,and the process is terminated.

Although the preferred embodiments of the invention have been disclosedfor illustrative purposes, those skilled in the art will appreciate thatvarious modifications, additions and substitutions are possible, withoutdeparting from the scope and spirit of the invention as disclosed in theaccompanying claims.

INDUSTRIAL APPLICABILITY

In the preferred embodiment of the present invention, a user oneselfre-initialization function is established in a portable gas sensor suchas a portable alcohol analyzer and a portable leaking gas sensor, and auser can conveniently maneuver the calibration operation mode by using agas which can be easily obtained, e.g., a human breath or air, as asubstitutionary reference gas. Typically, the semiconductor gas sensorhas reactivity to various gases such like CH based gas, H₂O and CO.However, the reactivity of each semiconductor gas sensor is slightlydifferent from each other depending on catalysis and temperature. Thisfact enables the semiconductor gas sensor to sense a particular gas.Based on this useful and unique characteristic of the semiconductor gassensor, the inventive portable gas sensor is developed. That is, abreath exhaled into the semiconductor gas sensor shows a similarreaction pattern to an alcohol concentration of about 10 PPM even thoughthere is a slight variation. For instance, since a typical human breathcan be considered to be a substitutionary reference gas of about 10 PPM,this type of gas is referred as the substitutionary reference gas in thepresent invention. Based on this fact, this type of breath is firstexhaled into the gas injecting unit of the portable gas sensor, and anew reference value is set by reading a resistance value of the sensingmaterial of the sensing unit in the portable gas sensor reacting to theinjected gas.

Meanwhile, in case of applying the preferred embodiment of the presentinvention to a leaking gas sensor for LPG or LNG, the gas sensor isrequired to be more sensitive compared to the alcohol analyzer.Therefore, it is preferable to take unpolluted air as a reference gasinstead of using the human breath. Since the unpolluted air has aconsistent composition ratio, it can be used as the relatively accuratereference gas.

Nevertheless, if a slope of a characteristic curve shown in FIG. 1 losesits linearity at a low concentration, the curve I could have differentresistance values like A, B and C. If the concentration is estimated inconsideration of these deviated resistance values, it is possible tosense a gas sensitively even if the characteristic of the gas sensor ischanged from the curve I to curve II or III.

Although the preferred embodiment of the present invention provides theexemplary portable alcohol analyzer and portable leaking gas sensor, itis still possible to apply the present invention to many other types ofa portable gas sensor.

Also, although use of a human breath or air is described in thepreferred embodiment of the present invention, any easily attainable gashaving a consistent composition ratio can be used as the substitutionaryreference gas for the calibration operation mode. Furthermore, aconvenient maneuver of the calibration operation mode by a user providesan effect on enhancing accuracy of the gas sensor.

1. A portable gas sensor having a sensing material reacting to a targetgas, comprising: a mode input unit for selecting an operation mode; agas injecting unit bringing in an external gas; a sensing unitoutputting a voltage value corresponding to a changed resistance valueof the sensing material due to a reaction between the sensing materialand the injected external gas; a first memory storing a referencevoltage value corresponding to a resistance value of the sensingmaterial with respect to a reference gas; a calibration control unit forstoring a first voltage value in the first memory by replacing thereference voltage value stored in the first memory through an operationof a calibration mode initiated in response to an input from the modeinput unit, the first voltage value corresponding to the changedresistance value of the sensing material reacting with a substitutionaryreference gas; a target gas sensing control unit for storing a secondvoltage value in a second memory by operating a target gas measurementoperation mode initiated in response to an input of the mode input unit,the second voltage value corresponding to a changed resistance value ofthe sensing material reacting with the target gas transferred throughthe gas injecting unit; a comparison and calculation unit for comparingthe first voltage value with the second voltage value and estimating thecomparison value; and a display unit displaying a concentration of thetarget gas estimated from the comparison and calculation unit.
 2. Theportable gas sensor as recited in claim 1, further comprising a thirdmemory for storing a lookup table for determining a concentration of thetarget gas depending on a value difference between the first voltagevalue and the second voltage value.
 3. The portable gas sensor asrecited in claim 1, wherein the substitutionary reference gas is a humanbreath or air/atmosphere.
 4. The portable gas sensor as recited in claim1, wherein the mode input unit selects an operation mode by using a timedifference in pressing a power button.
 5. A method for calibrating aportable gas sensor having a sensing material reacting to a target gas,comprising the steps of: storing a reference voltage value correspondingto a resistance value of the sensing material with respect to areference gas in a memory; inputting a calibration operation mode bymaneuvering a key operation; measuring a fist voltage corresponding to aresistance value of the sensing material with respect to asubstitutionary reference gas; and storing the first voltage value inthe memory by replacing the stored reference voltage value.
 6. Themethod as recited in claim 5, wherein the substitutionary reference gasis a human breath or air/atmosphere.
 7. A computer readable recordingmedium storing instructions for implementing the method for calibratingthe portable gas sensor having the sensing material reacting to thetarget gas, the computer readable recording medium comprising theinstructions of: storing a reference voltage value corresponding to aresistance value of the sensing material with respect to a referencegas; inputting a calibration operation mode by maneuvering a keyoperation; measuring a fist voltage corresponding to a resistance valueof the sensing material with respect to a substitutionary reference gas;and storing the first voltage value in the memory by replacing thestored reference voltage value.